1. Field of the Invention
This invention relates to an internal combustion engine control system, and more particularly to an internal combustion engine control system which controls at least one of the quantity of intake air and quantity of fuel supply to the engine and further controls the air-fuel ratio of the mixture.
2. Description of the Related Art
Electronic control for internal combustion engines is prevailing recently. Various types of controllers have been proposed with the intention of controlling at least one of the fundamental controlled variables of the internal combustion engine, i.e., the quantity of intake air or fuel supply, in accordance with the operational state of the engine, as described for example in Japanese Patent Unexamined Publication No. 56-107925. These engine controllers, when employing the air priority system in which the throttle valve opening is determined by the depth of accelerator operation, implement synchronous fuel injection with the quantity of fuel being determined properly basing on the quantity of intake air, engine coolant temperature and intake air temperature, and also modify the quantity of fuel injection or increase the fuel supply by asynchronous fuel injection depending on the operating condition. These modification operations take place at transitional events of engine operation such as an increased power demand at starting of the vehicle with its closed throttle valve being operated to open, recovery from the fuel-cut condition, and activation of the air conditioner. Even the fuel priority system determines the quantity of fuel supply first, and it also implements various modifications by anticipation of a possible transitional change in the engine operating condition.
There are also proposed several control system with the intention of enhanced exhaust detoxification and fuel economy by controlling the air-fuel ratio. Such engine control systems normally operate to detect the quantity of intake air to the engine and determine from it the quantity of fuel to be injected, and also control the quantity of fuel injection depending on the exhaust composition (generally the oxygen concentration in the exhaust gas is detected), thereby maintaining the intended air-fuel ratio, e.g., the stoichiometric air-fuel ratio (A/F .apprxeq.15) or the lean air-fuel ratio, associated with the specific engine operating condition, as described for example in Japanese Patent Unexamined Publication No. 54-57029.
However, the first-mentioned engine control systems which control at least the quantity of intake air and quantity of fuel supply implement modification for the controlled variable, i.e., quantity of air or fuel, on an inference basis to some extent at a transition, and therefore the modification does not always provide the expected result or even deteriorates the operational characteristics when adverse conditions arise coincidently or due to the disparity of quality and aging of component parts.
Also in the second-mentioned engine control system which controls the air-fuel ratio, the controlled variable could deviate from the intended air-fuel ratio at a transition of load condition or the like due to the delay from a change in the quantity of intake air until the quantity of fuel to be injected is calculated or the delay in detecting the oxygen concentration in the exhaust (after air intake, the engine operates for the compression stroke and combustion stroke before the oxygen concentration is measured in the exhaust). To cope with this matter, there has been proposed a control system which performs asynchronous fuel injection to modify the air-fuel ratio by anticipation, thereby improving the control response. Even in this case, the result of control appers in the exhaust after the engine has operated for the compression and combustion strokes, and it is finally detected in the exhaust stroke as a change in the oxygen concentration, thus giving rise to the same problem as in the case of the first-mentioned engine control systems.
For example, in the engine control system designed such that when the vehicle driver has applied the accelerator to race the engine from its idling state, the idle switch is operated by the opening throttle valve to increase the fuel by asynchronous fuel injection in anticipation of start-up, the fuel supply is increased as specified even if the throttle opening is little and, therefore, the amount of intake air does not change significantly, resulting in an excessively rich air-fuel ratio. In consequence, the engine output falls, and this can incur engine stall in the worst case. The same impropriety arises at asynchronous fuel injection when the vehicle is accelerated or it is restored from fuel-cut. Even with much ingeneous engine control which responds proportionally to events, it is difficult to completely get rid of the above-mentioned problems so far as there exist mechanical errors and disparities in the fuel injection valve, intake air metering device and idle switch.
In addition, the engine control system which controls the air-fuel ratio further have the following problems. Recent fuel injection control for internal combustion engines is elaborate and it employs a microcomputer to perform interrupt control for asynchronous fuel injection, fuel-cut, and the like depending on various conditions, thereby maintaining the quantity of fuel appropriately relative to the quantity of intake air. At the same time various electrical appliances including an air conditioner are often installed in the vehicle, causing the engine to operate in a wider range of condition and under a variety of combination of individual conditions. The failure of a sensor for detecting the engine operational state or its erroneous detection can present an operational state which would never occur usually. Although counter measures are taken against these cases, the air-fuel ratio can be unbalanced significantly on occasions of the specific combination of engine operating condition and the disparity and change by aging of component parts, which eventually incurs engine troubles such as engine stall.